1. Field of the Invention
This invention relates to electromagnetic shields for minimizing electromagnetic fields which interfere with the operation of electrical and magnetic devices.
2. Description of the Prior Art
Electromagnetic shields are used for minimizing and reducing to substantially zero electromagnetic fields which interfere with the operation of electrical and magnetic devices. For example, in color television receivers, the electron beam of the picture tube generally requires shielding from interference by 60 Hz fields produced internally by transformers or other electrical components. Shielding is also required from interference by external equipment, such as permanent magnets and electromagnets, since these also may adversely affect picture quality. Similarly, magnetic tape used in computers as well as for recording sound must also be shielded from stray fields.
Presently, such shielding usually consists of punched and drawn stampings from crystalline alloys. For example, where high permeability is required, alloys such as the 80/20 nickle-iron series (sold under the designation Mumetal, Hymu 80, Moly Permalloy) or the 50/50 nickle-iron series (4750 Alloy, Hipernik) are used. Where high saturation induction at moderate permeability is required, ingot iron or 3percent silicon iron is used.
For example, for protecting instrumentation from the earth's field, high permeability 80/20 nickel-iron alloy shields will reduce the field to less than one tenth of its original value (about 0.5 gauss in the U.S.A.). These shields are ideal for low field applications. However, if it should be desired to protect an aircraft compass system from the large stray fields (about 20 to 30 gauss) produced by devices containing permanent magnets, such as d'Arsonval ammeters and voltmeters, magnetrons, magnetic recorders, permanent magnet motors and the like, then ingot iron sheet or silicon iron sheet with their higher saturation induction are wrapped around the stray field source to absorb most of the stray flux. If additional shielding is desired, present practice adds a high permeability nickle-iron shield about 1/4 inch away from the ingot iron to further reduce the field to extremely low values.
The shielding materials commonly employed in commercial practice suffer from several major drawbacks. First, all alloys must be heat treated, usually in pure dry hydrogen, at temperatures above 800.degree. C to achieve optimum magnetic properties. Second, after this costly annealing, any slight bending strain will substantially lower the maximum permeability. Further, when these crystalline materials are formed into complex shapes, they must be reannealed at elevated temperatures to regain their outstanding magnetic properties. However, such reannealing does not permit close dimensional tolerances to be maintained. As a consequence, flexible shields having desirable magnetic properties are not readily available.